The invention relates to methods and apparatuses for the monitoring deposition from an aqueous solution on the surfaces of process equipment. More specifically, the invention relates to a method of monitoring scale deposition from black liquor in pulp mill digesters, evaporators and concentrators. The invention has particular relevance to monitoring and inhibiting scale deposition in pulp mill digesters, evaporators and concentrators to improve process efficiency in pulping operations.
The kraft pulping process is one of the major pulping processes in the pulp and paper industry. Spent liquor resulting from the kraft pulping process (black liquor or “BL”) contains various organic materials as well as inorganic salts, the deposition of which detracts from an efficient chemical pulping and recovery cycle due to decreasing thermal conductivity and lengthy boilouts. Other, less common pulping processes such as alkaline or sulfite pulping differ from the kraft process in the chemical composition of the liquor that affect the composition of the deposits.
The most common scale in the pulp and paper industry is calcium carbonate that is also a prevalent scale in many other industries (Amjad, Z. (Ed.) Mineral scale formation and inhibition. Plenum, N.Y. (1995); Cowan, J. C., Weintritt, D. J. Water-Formed Scale Deposits. Gulf Publ. Co., Houston, Tex. (1976)). It is an especially severe problem in kraft digesters (Markham, L. D., Bryce, J. R. G., Formation of calcium carbonate scale in a Kamyr digester, Proc. TAPPI Pulping Conf., TAPPI Press, 17-21 (1979); Severtson, S. J., Duggirala, P. Y., Carter, P. W., Reed, P. E. Mechanism and chemical control of CaCO3 scaling in the kraft process. TAPPI J., 82(6), 167-174 (1999); Sitholé, B. Scale deposit problems in pulp and paper mills. Proc. African Pulp and Paper Week, Durban, S A (2002)). Normally, it is not possible to fully inhibit the precipitation of calcium carbonate due to high driving force. Successful approaches to inhibiting deposition rely on making the deposit non-adherent and dispersed, or sequestration of the calcium ions. Calcium carbonate deposition can be temperature- or pH-induced. While temperature induction is typical of digesters and evaporators, at the bleach plant calcium carbonate scale is more often induced by alkalization. A wide spectrum of calcium carbonate treatment products is available, because their relative performance depends on the conditions, and the applicability depends on the issues of stability and environmental regulations.
Calcium carbonate deposits form extensively at many stages of the papermaking process. As described for example in U.S. Pat. Nos. 7,985,318, 6,053,032, 6,942,782, 6,250,140, and 5,734,098, inorganic salt scaling in spent liquor evaporators and concentrators continues to be one of the most persistent problems encountered in the pulp and paper industry. Concentrated liquor contains calcium, sodium, carbonate, and sulfate ions at levels high enough to form scales that precipitate from solution and deposit on heated surfaces. The most important types of scale in evaporators are hard scale, such as calcium carbonate (CaCO3), and soft scale, such as burkeite (2(Na2SO4):Na2CO3). The solubility of both types of scale decreases as temperature increases, which causes the scales to adhere to heat transfer surfaces thus drastically reducing the overall efficiency of the evaporator (See Smith, J. B. & Hsieh, J. S., Preliminary investigation into factors affecting second critical solids black liquor scaling. TAPPI Pulping/Process, Prod. Qual. Conf., pp. 1 to 9, 2000 and Smith, J. B. & Hsieh, J. S., Evaluation of sodium salt scaling in a pilot falling film evaporator. TAPPI Pulping/Process, Prod. Qual. Conf., pp. 1013 to 1022, 2001; and Smith, J. B. et al., Quantifying burkeite scaling in a pilot falling film evaporator, TAPPI Pulping Conf., pp. 898 to 916, 2001).
Generally, monitoring of inorganic scale is most efficiently achieved using quartz crystal microbalance (“QCM”) based technologies. Applicability of QCM-based instruments is determined, however, by sensor crystal stability under process conditions. Such instruments cannot be used under high temperature and/or high alkalinity conditions. This limitation makes the technology useless in digesters and evaporators. Besides a simple gravimetric technique and a non-quantitative characterization using Lasentec-FBRM, a technique based on deposit accumulation on the heated surface was proposed for liquors with solid content higher than 55% (“Method of monitoring and inhibiting scale deposition in pulp mill evaporators and concentrators,” U.S. Pat. No. 8,303,768. However, this method is capable of detecting only sizable volumes of the deposit that makes it unusable in kraft mill digesters where calcium carbonate accumulation is too slow for the method of such a low sensitivity.
There thus exists an ongoing need to develop sensitive methods of monitoring deposits in the pulp and paper industry under the conditions of high temperature, alkalinity and pressure. Such monitoring is of particular importance in pulp mill digesters, evaporators and concentrators. The art described in this section is not intended to constitute an admission that any patent, publication or other information referred to herein is “prior art” with respect to this invention, unless specifically designated as such. In addition, this section should not be construed to mean that a search has been made or that no other pertinent information as defined in 37 CFR §1.56(a) exists.